Overtraining or Under-Fuelling? How to Tell the Difference

Overtraining and under-fuelling produce almost identical symptoms, because both start with the same energy shortfall in the brain. The difference is that overtraining syndrome is a diagnosis of exclusion, meaning low energy availability has to be ruled out first. When researchers reviewed every training overload study that had also measured what athletes were eating, 18 of the 21 studies (86%) showed reduced energy or carbohydrate availability during the overload period (Stellingwerff et al., 2021, Sports Medicine). Before you add another session, check three things: whether your appetite has scaled with your training, what the scales are doing alongside your performance, and whether your top end has gone while your easy sessions still feel fine. 

Your legs are heavy and they never quite clear. Sessions that look easy on paper feel brutal. You are not improving, despite training consistently for months. Your sleep has gone off, your mood swings around, you keep catching whatever is going around, and every little niggle hangs about for weeks longer than it should.

If you read that list and thought overtraining, you are right. Here is the part almost nobody tells age-group triathletes: that is also the symptom list for chronic under-fuelling. Same list, two completely different causes, two completely different fixes.

I see this constantly. Athletes turn up to work with me having done everything right for 18 months without progressing for a single one of them. They do not lack discipline. They are doing things the hard way, because they have been solving the wrong problem.

Why Do Overtraining and Under-Fuelling Cause the Same Symptoms? 

Because they both start in the same place. Both trace back to the hypothalamus and the pituitary (the control panel in your brain that decides how much energy your body is allowed to spend on anything beyond immediate survival). Your hormones, your bone density, your immune system and your ability to build fitness are all funded out of what is left over.

That control panel does not ask why the energy is short. It does not care whether you trained too much or ate too little. It registers that there is not enough to go around, and it starts turning dials down. Same origin, same dials, same symptoms.

This is not a fringe theory. A review of the shared pathways between overtraining syndrome and Relative Energy Deficiency in Sport concluded that many of the negative outcomes attributed to training overload may be primarily due to misdiagnosed under-fuelling (Stellingwerff et al., 2021, Sports Medicine). Of the 21 training overload studies that had also measured energy intake, carbohydrate, exercise energy expenditure or energy availability, 18 showed a drop.

One more thing worth understanding. You do not get fitter during your sessions. The session is only the stimulus. The actual adaptation happens afterwards in recovery, and recovery costs energy to run. If the energy is not there, you send the signal and nothing gets built. You paid the deposit and the builder never turned up.

What Is Energy Availability, and Why Is It Not Calories In Versus Calories Out?

Energy availability is what your body has left to run itself on, after it has paid for your training.

Take everything you eat in a day, subtract what your training cost you, and whatever is left has to fund every other process you have going: beating your heart, making hormones, keeping your bones dense, fighting off that cold going round the office, and building the adaptations from the session you just did. We express that relative to your fat free mass (your body minus your fat stores, so muscle and bone).

Below roughly 30 kcal per kg of fat free mass per day is where things go wrong. Around 45 kcal per kg is where things run properly. It is a scale, not a hard line, but those numbers matter.

Here is what that looks like on a real athlete. A 75 kg male, fat free mass 66 kg, training 12 hours a week. Not elite, just a committed age-grouper with a job, a family and a 70.3 on the calendar, trying to lean up for it. At roughly 600 kcal an hour, 12 hours costs him a bit over 1,000 kcal a day. Say he eats 2,800 calories (around 11,700 kJ, and on a plate that genuinely looks like a lot). He would tell you he eats plenty. He might tell you he eats too much.

Run the sum and he is at 26.8 kcal per kg fat free mass, under the line every day for months, with no idea. To sit at 45 he would need closer to 4,000 calories a day. A 69 kg female with 50 kg of fat free mass doing the same 12 hour week needs around 3,200 kcal, and eating just over 2,000 while chasing body fat loss puts her at 21.

Those numbers are simplified to show you the concept, and yours will be different. Body composition, session intensity, metabolism, medical history and about ten other things move them. This is exactly why I bang on about n=1. You are your own experiment, and nobody should be handing you a number off the internet or out of AI and calling it personalised.

Why Adding Training Volume Makes Under-Fuelling Worse

Your entire model for getting better at something is to do more of it. Weak swim, add swim sessions. That instinct has served you well, and it is the reason you can swim, bike and run all together in the first place. But you have aimed it at the wrong system.

If you are under-fuelled and you add a session, you have not added training. You have added cost. There is more work to recover from and no extra energy to recover with, so you feel worse, which looks like more evidence that you are unfit, which makes adding another session feel even more justified. Round it goes, and every lap digs the hole deeper.

Four studies have looked at what happens when you increase training volume by 20 to 50% and simply let athletes eat to appetite (Melin et al., 2024, Scandinavian Journal of Medicine & Science in Sports). Intake does not keep up. Energy availability falls off a cliff without a single conscious decision being made, because appetite is a rubbish indicator of what your body needs when training load is high. Nobody sits down in January and decides to spend the year under-fuelling. It just happens.

How Do You Tell If You Are Overtraining or Under-Fuelling? 3 Signals

The order of operations matters, and this is where age-group triathlon gets it backwards. Overtraining syndrome is a diagnosis of exclusion, so everything else that could explain the same picture has to be ruled out before anyone writes it down as the answer, and low energy availability sits at the top of that list (Meeusen et al., 2013, Medicine & Science in Sports & Exercise). The professionals check the fuelling first, then they consider the training. Most triathletes do it the other way around.

Signal 1: your appetite has gone the wrong way. If your training volume has climbed and your hunger has dropped, or just stayed where it was, that is backwards. Hard training and chronic under-fuelling both suppress appetite, because your body is trying to make what it has last longer. So the athlete who most needs to eat more is the one who feels like eating less. If you finished a three hour ride and genuinely were not hungry afterwards, that is not discipline. That is the feather, the first of three warning signs that get louder. Ignore the feather and your body throws a brick. Ignore the brick and the truck turns up, and the truck looks like a stress fracture, a written-off season, or a set of blood results that takes months to fix.

 Signal 2: the scales are not behaving. Version one, you added five hours a week through a big block and the number has not moved a gram. That is not always a plateau. It can be your body defending itself by down-regulating everything, and one of the first things to go is your resting metabolic rate. Version two, your weight is drifting down and your performance is drifting down beside it. When those two lines move together, you are very likely losing muscle, which is the exact opposite of what you were trying to do.

 Signal 3: easy is fine, but your top end has gone. The most nutrition-specific of the three. Zone 2 ticks along normally, then you go to hold threshold and it simply is not there, and a number you could hit three months ago is pure fantasy. That is low carbohydrate availability. Your aerobic system runs happily on fat at low intensity, but the moment you crank the intensity your body wants carbohydrate (stored in your muscles and liver as glycogen), and if the tank is empty there is nothing to give you.

There is a fourth sign worth watching, and it is the one athletes normalise: colds that keep coming back, and soft tissue niggles stacking on top of each other. Your immune system is expensive to run, and it is one of the first departments to have its budget cut when energy is tight. I went deeper on that pattern in Episode 121, Female Athlete Series Part 3: Relative Energy Deficiency, and there is more on the health consequences in this article on Relative Energy Deficiency in Sport.

If you ticked all three, I would be surprised if the thing capping your performance right now was your training program.

Overtraining or Under-Fuelling: What to Fix First

Three things, today, before you touch your training schedule.

Fix 1: fuel inside your sessions, not just around them. If you head out for a three hour ride on a bottle of water and a banana, do not be surprised that you are wrecked for the rest of the day. Under-fuelled long rides are energy borrowed against tomorrow, and the interest rate is brutal, particularly on your immune system. On the bike you have no excuse, because your stomach is not jiggling around the way it is when you run. Treat the bike as your rolling buffet.

Fix 2: have an actual plan for after training. This is the 4 Rs. Refuel with carbohydrate, repair with protein, rehydrate with fluid, and revitalise with some colour on your plate. If you get home, shower, do the school run and sort yourself out at some point later, your recovery nutrition needs work. Every session you do not recover from properly is a session you paid for and never collected on. I covered the basics in Episode 4, Stop Ditching Your Recovery Nutrition.

Fix 3: make your food move when your training moves. If your intake looks roughly the same on a rest day and a five hour day, that is set and forget, and set and forget is the one thing you never do with your nutrition as a triathlete. A periodised training week needs a periodised eating plan. Big day, big food. Light day, light food. If your training is a roller coaster and your food is a flat line, you are guaranteed to be over-fuelled on some days and badly under-fuelled on others, and the under-fuelled ones are almost always the days that matter most. Fuel for the work required. 

One important note. If something here has touched a nerve, and by that I mean food feels like a fight, or you are restricting or punishing yourself, or you can see a pattern bigger than knowing your numbers, please go and talk to somebody who specialises in that. I am not an eating disorder dietitian. That is a genuine specialty and it is not mine, so I will always refer you on to somebody who can help properly.

Frequently Asked Questions

Q: Can you be overtrained and under-fuelled at the same time?

A: Yes, and it is common. The two feed each other, because under-fuelling reduces your capacity to recover, which means the same training load becomes an overload. This is why the order of assessment matters. Ruling out low energy availability first tells you whether the training load was ever the real issue.

Q: What is a low energy availability number for a triathlete?

A: Below roughly 30 kcal per kg of fat free mass per day is where physiological function starts being compromised, and around 45 kcal per kg per day is where things run properly. It is a scale rather than a hard cut-off, and your individual number depends on body composition, training load, metabolism and medical history.

Q: How long does it take to under-fuel badly enough to cause problems?

A: Not long at all. Within a few days of low energy availability you can affect hormone function. For a female athlete that can mean losing a menstrual cycle that month. Male athletes do not get that obvious signal, but testosterone can be affected on a similar timeframe.

Q: Why am I not hungry after long training sessions?

A: Both hard training and chronic under-fuelling suppress appetite. Your body turns the hunger signal down to make the energy it has last longer. It feels like discipline, but it is a warning sign, and it means the athlete who most needs to eat more is the one who feels like eating least.

Q: Should I take a rest week or eat more first?

A: Check the fuelling first. Overtraining syndrome is a diagnosis of exclusion, so low energy availability has to be ruled out before training load takes the blame. A rest week on top of chronic under-fuelling will make you feel better temporarily and will not fix the underlying shortfall.

Bottom Line

Adding volume to an under-fuelled athlete does not make them fitter. It makes them more tired, more grumpy, slower, and considerably more expensive to fix in both time and money. If your appetite has gone backwards, the scales are not behaving and your top end has disappeared, your nutrition is what is capping your performance right now, not your training program.

Before you add that extra session, go and run the sums instead. It takes a couple of minutes and it might save your season.

And if you have read all of that and thought you have no idea what your number is, let alone how to fix it, do not stress. Nobody is born knowing how to fuel three sports, you will not learn it at school, and you do not know what you do not know. This is exactly what I teach inside the Triathlon Nutrition Academy: how to eat for training, racing and life, so you are not asking the internet random questions at 9pm every time your program changes. Get your name on the list for the next cohort here.